The present invention relates to a new and improved method and apparatus for shaping the edges of flow control lands of a valve core, and more specifically to a method and apparatus for forming fluid flow metering notches in the lands of a valve core.
Power steering control valves of the type disclosed in U.S. Pat. Nos. 3,709,099 and 3,921,669 have a valve core which is rotatable relative to a valve sleeve. The valve core has axially extending lands with axially extending grooves between the lands. The lands and grooves in the valve core cooperate with passages in the sleeve to direct fluid flow to and from a power steering motor. Metering notches have been formed in the edges of the lands in the valve core to provide a metered flow of fluid under pressure through the valve. The metering notches provide a variation in the flow area through the valve to provide the desired power assist characteristics.
The metering notches must be accurately formed relative to the lands. Even slight inaccuracies in forming the metering notches relative to a land results in a different response of the power steering motor upon actuation in one direction than the response which is obtained upon actuation in the other direction.
Prior art methods and apparatus for forming metering notches in valve cores are disclosed in U.S. Pat. Nos. 4,100,785 and 4,103,407 and in German Pat. No. 3,137,367. Typically, the valve core is positioned by a locator, and then the metering notches are simultaneously formed in the edges of a plurality of lands of the valve core by a plurality of tools which are simultaneously brought into engagement with the edges of the lands. Due to manufacturing tolerances, one or more of the lands may be slightly displaced from their intended position relative to other lands. Thus, the metering notch formed on one side of a displaced land may be slightly wider than the intended width of the metering notch, while the metering notch on the other side of the land may be slightly narrower than its intended width. Due to the difference in the configuration of the metering notches on opposite sides of the land, the response of the power steering motor to rotation of the valve core in opposite directions will be somewhat different. In high pressure hydraulic fluid flow control, small differences in flow area can produce large differences in result. Hence, extreme accuracy in forming flow control edges is critical.